Propellants derived from crosslinking of polybutadiene elastomers

ABSTRACT

Composite propellant compositions are disclosed which contain binders formed from polybutadiene prepolymers cross-linked by siloxane compounds. The crosslinking by siloxanes to produce propellant binders eliminates the need for end-capping polybutadiene to introduce functional groups through which crosslinking can be effected. The pertinent siloxane compounds contain two silicon hydride bonds of the types present in: 1,1,3,3-tetramethyldisiloxane, (CH3)2H.SiO.SiH(CH3)2; 1,1,1 3,5,7,7,7-octamethyltetrasiloxane, (CH3)3.Si.O.Si(H.CH3).O.Si(H.CH3).O.Si(CH3)3. The prepolymers are crosslinked in a composite propellant composition to produce the propellant binder for the composition. When the crosslinking technique is used in a propellant production process which additionally employs hydroxyl-terminated polybutadiene in combination with a polyisocyanate, a propellant binder with improved mechanical properties and improved stability is produced.

14 1 Feb. 6, 1973 l l PROPELLANTS DERIVED FROM CROSSLINKING OFPOLYBUTADIENE ELASTOMERS {75] Inventor: David C. Sayles, Huntsville,Ala.

1731 Assignec: The United States of America as represented by theSecretary of the Army [22] Filed: May 12, I970 {21] Appl. No.: 48,618

3,551,225 l2/l970 Saylcs ..l49/76X Primary ExaminerStephen J. Lechert,Jr. Attorney-Harry M. Saragovitz, Edward J. Kelly, Herbert Berl andJames T. Deaton [57 I ABSTRACT Composite propellant compositions aredisclosed which contain binders formed from polybutadiene prepolymerscross-linked by siloxane compounds. The V crosslinking by siloxanes toproduce propellant binders eliminates the need for end-cappingpolybutadienc to introduce functional groups through which crosslinkingcan be effected. The pertinent siloxane compounds contain two siliconhydride bonds of the types present in: l,l,3,3-tetramethyldisiloxane,(CH H-SiO'SiH(CH )Z; 1,l ,l 3 ,5 ,7 ,7 ,7-octamethyltetrasiloxane, (CH'Si'O-Si(H'CH )'O-Si(H-CH )O -Si(CH The prepolymers are crosslinked in acomposite propellant composition to produce the propellant binder forthe composition. When the crosslinking technique is used in a propellantproduction process which additionally employs hydroxyl-terminatedpolybutadiene in combination with a polyisocyanate, a propellant binderwith improved mechanical properties and improved stability is produced.

8 Claims, No Drawings PROPELLANTS DERIVED FROM CROSSLINKING OFPOLYBUTADIENE ELASTOMERS CROSS-REFERENCE TO RELATED APPLICATION Thisapplication is related to application Ser. No. 85 l ,l 37, filed July30, 1969, in the use of an interfacial bonding agent disclosed therein.

BACKGROUND OF THE INVENTION 7 Prior art composite propellantcompositions have employed binders of the polybutadiene type, bothhydroxyl-or carboxyl-terminated. When polybutadiene is terminated by ahydroxyl-or carboxyl-group, a functional group is thereby introducedthrough which crosslinking is effected. The process for terminatingpolybutadiene is generally referred to as end-capping. One method forend-capping or the production of hydroxylor carboxyl-terminatedpolybutadiene is by anionic polymerization of butadiene using a metalalkyl. The terminal functional groups are attached by reaction with anepoxide or carbon dioxide respectively to produce polybutadieneprepolymers. The described polybutadiene prepolymers are crosslinked bymeans of polyisocyanates for the hydroxyl-terminated prepolymers or bymeans of epoxides or aziridinyl compounds for the carboxyl-terminatedprepolymers. The polyurethane, the phosphamide, and the polyester groupsof these polymers are susceptible to hydrolytic degradation or scissionby various types of reactions. Degradation and/or scission of groupswithin propellant systems are detrimental to the propellant's desirablephysical and chemical properties.

7 An object of this invention is to provide a composite propellant whichutilizes a binder formed from polybutadiene prepolymer crosslinked witha siloxane.

Another object of this invention is to provide a composite propellantwhich utilizes a binder formed from hydroxyl-terminated polybutadienecrosslinked with a siloxane and a polyisocyanate.

SUMMARY OF THE INVENTION Polybutadiene prepolymer is crosslinked with asiloxane to form a binder for the propellant composition wherein used.The point of reaction is with the unsaturated linkages which occur inthe polybutadiene prepolymer one per four carbon atoms. The reactionbetween a terminal alkenic linkage of two moieties of two polybutadieneprepolymers and the l,l,3,3- tetramethyldisiloxane is depicted by thefollowing structural equations:

Product (A) represents a portion of the binder material wherein n equalsmultiple units dependent upon the molecular weight of the prepolymeremployed in the reaction. The catalyst for the reaction may be selectedfrom chloroplatinic acid, iron linoleate, and the like.

The composite propellant compositions of this invention are comprised ofthe ingredients (in weight percent of the propellant composition) asfollows: inorganic oxidizer ammonium perchlorate in amounts from about60-72 weight percent, aluminum metal fuel in amounts from about 5 toabout 20 weight percent, polybutadiene prepolymer from about 8 to about16 weight percent, a siloxane selected from l,l,3,3-tetramethyldisiloxane and 1,1,l,3,5,7,7,7-octamethyltetrasiloxane fromabout 0.3 to about 2.0 weight percent, n-butylferrocene (burning ratecatalyst) from about 0.5 to about 3.0 weight percent, and iron linoleatefrom about 0.1 to about 0.3 weight percent. An interfacial bonding agentin an amount from about 0.05 to about 1.0 weight percent of the totalpropellant ingredients, as an additive, may be used in the composition.

Powdered metals, as fuels, useful in the propellant compositions of thisinvention include in addition to aluminum, magnesium, titanium,zirconium, and boron. Alloys and mixtures of the aforesaid metals mayalso be employed. Aluminum metal powder is the preferred metal fuel.

The interfacial bonding agents employed in the propellant formulationsof this invention may be any of the reaction products of diortri-functional aziridinyl phosphine oxides or their derivatives withpolyfunctional carboxylic acids as specifically set forth in a commonlyassigned Patent Application Ser. No. 85l,l37, filed July 30, 1969. Thenominal structure of the reaction product may be represented by thefollowing general formula:

Where X represents an aziridine group of the structure:

and Q and 0 are either hydrogen or alkyl groups of one to four carbonatoms (0, and 0 may be the same or different), X, may be the same as X,or may be an organic radical such as phenyl, benzyl, ethyl, etc., R isan alkyl that contains at least one active hydrogen atom or an organicentity of molecules that contain one or more hydroxyl groups, and n is2, 3, or 4.

The reaction product describedabove, is produced by dissolving thereactants in a suitable inert organic solvent such as methanol, ethanol,methylene chloride, tetrahydrofuran, diethyl ether, or mixtures ofthese. It has been found to be preferable that methanol or ethanolcomprise at least a part of the solvent. Reaction temperature is notcritical, and may range from to 200F for such time as is needed foressentially all carboxyl groups to be reacted. The solvent is thenremoved by any suitable means, such as evaporation under vacuum atelevated temperatures. The residue is the reaction product, aninterfacial bonding agent,v

which is usually straw-colored and quite viscous.

DESCRlPTION OF THE PREFERRED EMBODIMENTS The disiloxanes preferred foruse in this invention have two silicon hydride bonds as describedearlier herein. The 1.,1,3,3-tetramethyldisiloxane has the followingother characteristics: b.p. 72C at 760 mm of- 67/33 blend of 200p.l20p.particle size is preferred;

however, different burning rates can be obtained and different physicalproperties can be obtained by other 7 combinations ofdifferent particlesizes.

Aluminum powder of 6p particle size is very satisfactory for use in thisinvention.

The manufacture of a composite propellant wherein the polybutadieneprepolymer is crosslinked with a polysiloxane can be effected using theusual equipment which is available for the preparation of compositepropellants.

A representative composition of a propellant of this invention is setforth under Example 1 below.

EXAMPLE 1 Weight Percent Ingredient Ammonium perchlorate (67/33-'200p.l20p.71.0

An additive of an interfacial bonding agent in the amount of 0.1 weightpercent, (based on the weight of other propellant ingredients), is addedto the composition to improve mechanical properties of the curedpropellant.

The interfacial bonding agent, such as the reaction products of diortri-functional aziridinyl phosphine oxides or their derivatives withpolyfunctional carboxylic acids and described earlier herein, isbelieved to function by coating the ammonium perchlorate with a thinfilm of the described product. The ammonium perchlorate then catalyzesthe homopolymerization of the remaining aziridine rings in the reactionproduct, thus linking the molecules together to yield a highlycross-linked network product.

Example 11 illustrates how the crosslinking technique of the disiloxanecan be used with presently used binder materials to render improvementof properties thereof. l-lydroxylterminated polybutadiene is used incombination with a typical polyisocyanate as the crosslinking agent, andthe polybutadiene prepolymer is used with the disiloxane as acrosslinking agent. A highly crosslinked polymer results from thecombination. In addition to providing crosslinking for terminal alkeniclinkage of the polybutadiene of the prepolymer, the disiloxane providesadditional crosslinking for the binder system formed fromhydroxyl-terminated polybutadiene crosslinked with a polyisocyanate.Example 11 illustrates how a polybutadiene prepolymer and a disiloxanecan be utilized with a conventional end-capped polybutadiene andcrosslinking agent to provide a high binder content propellant withimproved mechanical properties and improved stabilities againsthydrolytic degradation and scission by various types of reactions.

The range of percent by weight of the propellant set forth earlierherein applies to the propellant ingredients of Example 11. Theadditional ingredients set forth in Example 11 (e.g., hydroxylterminatedpolybutadiene and toluene diisocyanate) are used in amounts from about 5to about 10 percent by weight of the propellant composition and fromabout 0.5 to about 2 percent by weight of the propellant composition,respectively.

Also, the polybutadiene prepolymer, when used in combination withhydroxyl-terminated polybutadiene, is used in amounts from about 5 toabout 10 percent by weight of the propellant composition.

EXAMPLE ll INGREDIENT Ammonium perchlorate (67/33-200pt/20p.)

WEIGHT PERCENT The interfacial bonding agent as used under Example 1 maybe optionally used with Example 11 formulation in an amount from about0.05 weight percent to about 1.0 weight percent of the other propellantingredients. The interfacial bonding agent is used as previouslydescribed to improve mechanical properties and stability of the finishedpropellant.

1 claim:-

I. A composite propellant composition comprising an inorganic oxidizerammonium perchlorate; aluminum metal fuel; a burning rate catalystn-butylferrocene; and a binder comprised of a polybutadiene prepolymercrosslinked under catalytic reaction with a selected siloxane compoundcontaining two silicon hydride bonds, said catalytic reaction beingconducted in the presence of a catalyst selected from the groupconsisting of chloroplatinic acid and iron linoleate and said siloxanecompound being selected from the group consisting ofl,l,3,3-tetramethyldisiloxane and 1 ,1,1,3 ,5 ,7,7,7-octamethyltetrasiloxane.

2. The propellant composition of claim 1 and wherein said inorganicoxidizer ammonium perchlorate is present in amounts from about 60 toabout 72 weight percent of said composition; said aluminum metal fuel ispowdered and said aluminum metal fuel is present in amounts from about 5to about 20 weight percent of said composition; said burning ratecatalyst of n-butyli'crrocene is present in amounts from 0.5 to about3.0 weight percent of said composition; said catalyst for catalyticreaction is iron linoleate which is present in amounts from about 0.1 toabout 0.3 weight percent of said composition; said selected siloxanecompound is present in an amount from about 0.3 to about 2.0 weightpercent of said composition; and wherein said composition additionallycontains an interfacial bonding agent, as an additive, in the amount offrom about 0.05 to about 1.0 weight percent of the total propellantingredients.

3. The propellant composition of claim 2 and wherein said inorganicoxidizer ammonium perchlorate is a blend of 200 micron particle size, 67parts, and 20 micron particle size, 33 parts, said blended ammoniumperchlorate being present in an amount of about 71 weight percent ofsaid composition; said aluminum metal fuel is present in an amount ofabout 16 weight percent of said composition; said burning rate catalystof n-butylferrocene is present in an amount of about 2.5 weight percentof said composition; said iron linoleate is present in an amount ofabout 0.1 weight percent of said composition; said binder is present inan amount of about 10.4 weight percent of said composition said binderbeing comprised of said polybutadiene prepolymer in an amount of aboutweight percent of said composition and crosslinked with the selectedsiloxane compound which is 1,1 ,3,3-tetramethyldisiloxane, and saidselected siloxane compound being present in an amount of about 0.4weight percent of said composition; and wherein said interfacial bondingagent is present in an amount of about 0.1 weight percent of the totalpropellant ingredients.

4. The propellant composition of claim 2 and wherein said inorganicoxidizer ammonium perchlorate is a blend of 200 micron particle size, 67parts, and 20 micron particle size, 33 parts; said aluminum metal fuelis present in an amount of about 16 weight percent of said composition;said burning rate catalyst of n-butylferrocene is present in an amountof about 2.5 weight percent of said composition; said iron linoleate ispresent in an amount of about 0.1 weight percent of said composition;said binder is present in an amount of about 10.4 weight percent of saidcomposition, said binder is comprised of said polybutadiene prepolymerin an amount of about 10 weight percent of said composition andcrosslinked with the selected siloxane compound which is1,1,l,3,5,7,7,7-octamethyltetrasiloxane, said selected siloxane compoundbeing present in an amount of about 0.4 weight percent of saidcomposition; and wherein said interfacial bonding agent is present in anamount of about 0.1 weight percent of the total propellant ingredients.

5. The composite propellant composition of claim 1 wherein saidcomposition contains an additional binder of hydroxyl-terminatedpolybutadiene crosslinked with toluene diisocyanate.

6. The propellant composition of claim 5 and wherein said inorganicoxidizer ammonium perchlorate is present in amounts from about 60toabout 72 weight percent of said composition; said aluminum metal fuelis powdered and said aluminum powder is present in amounts from about 5to about 20 weight percent of said composition; said burning ratecatalyst of n-butylferrocene is present in amounts from about 0.5 toabout 3.0 weight percent of said composition; said iron linoleate ispresent in amounts from about 0.1 to about 0.3 weight percent of saidcomposition; said hydroxylterminated polybutadiene is present in amountsfrom about 5 to about 10 percent by weight of said composition; saidtoluene diisocyanate is present in amounts from about 0.5 to about 2.0percent by weight of said composition; said polybutadiene prepolymer ispresent in amounts from about 5 to about 10 percent by weight of saidcomposition; said selected siloxane compound is selected from the groupconsisting of l,l,3,3- tetramethyldisiloxane and1,1,1,3,5,7,7,7-octamethyltetrasiloxane and is present in amounts fromabout 0.3 to about 2.0 percent by weight of said composition; and saidcomposition additionally containing an interfacial bonding agent, as anadditive, in the amount of from about 0.05 to about 1.0 percent byweight of the total propellant ingredients.

7. The propellant composition of claim 6 and wherein said inorganicoxidizer ammonium perchlorate is a blend of 200 micron size, 67 parts,and 20 micron particle size, 33 parts, said blended ammonium perchloratebeing present in an amount of about 66 percent by weight of saidcomposition; said aluminum metal fuel is present in an amount of about16 percent by weight of said composition; said burning rate catalyst ofn-butylferrocene is present in an amount of about 2.5, percent by weightof said composition; said iron linoleate is present in an amount ofabout 0.1 percent by weight of said composition; said polybutadieneprepolymer is present in an amount of about 7 percent by weight of saidcomposition; said selected siloxane compound isl,1,3,3-tetramethyldisiloxane which is present in an amount of about 0.4percent by weight of said composition; said hydroxyl-terminatedpolybutadiene is present in an amount of about 7 percent by weight ofsaid composition; said toluene diisocyanate is present in an amount ofabout 1.0 percent by weight of said composition; and said interfacialbonding agent is present in an amount of about 0.1 percent by weight ofthe total propellant ingredients.

8. The propellant composition of claim 7 and wherein said inorganicoxidizer ammonium perchlorate is a blend of 200 micron size, 67 parts,and 20 micron particle size, 33 parts, said blended ammonium perchloratebeing present in an amount of about 66 percent by weight of saidcomposition; said aluminum metal fuel is present in an amount of about16 percent by weight of said composition; said burning rate catalyst ofn-butylferrocene is present in an amount of about 2.5 percent by weightof said composition; said iron linoleate is present in an amount ofabout 0.1 percent by weight of said composition; said polybutadieneprepolymer is present in an amount of about 7 percent by weight of saidcomposition; said selected siloxane compound is 1 ,1 ,l,3,5,7,7,7-octamethyltetrasiloxane which is present in an amount ofabout 0.4 percent by weight of said composition; saidhydroxyl-terminated polybutadiene is present in an amount of about 7percent by weight of said composition; said toluene diisocyanate ispresent in an amount of about 1.0 percent by weight of said composition;and said interfacial bonding agent is present in an amount of about 0.1percent by weight of the total propellant ingredients.

1. A composite propellant composition comprising an inorganic oxidizerammonium perchlorate; aluminum metal fuel; a burning rate catalystn-butylferrocene; and a binder comprised of a polybutadiene prepolymercrosslinked under catalytic reaction with a selected siloxane compoundcontaining two silicon hydride bonds, said catalytic reaction beingconducted in the presence of a catalyst selected from the groupconsisting of chloroplatinic acid and iron linoleate and said siloxanecompound being selected from the group consisting of1,1,3,3-tetramethyldisiloxane and 1,1,1,3,5,7,7,7-octamethyltetrasiloxane.
 2. The propellant composition ofclaim 1 and wherein said inorganic oxidizer ammonium perchlorate ispresent in amounts from about 60 to about 72 weight percent of saidcomposition; said aluminum metal fuel is powdered and said aluminummetal fuel is present in amounts from about 5 to about 20 weight percentof said composition; said burning rate catalyst of n-butylferrocene ispresent in amounts from 0.5 to about 3.0 weight percent of saidcomposition; said catalyst for catalytic reaction is iron linoleatewhich is present in amounts from about 0.1 to about 0.3 weight percentof said composition; said selected siloxane compound is present in anamount from about 0.3 to about 2.0 weight percent of said composition;and wherein said composition additionally contains an interfacialbonding agent, as an additive, in the amount of from about 0.05 to about1.0 weight percent of the total propellant ingredients.
 3. Thepropellant composition of claim 2 and wherein said inorganic oxidizerammonium perchlorate is a blend of 200 micron particle size, 67 parts,and 20 micron particle size, 33 parts, said blended ammonium perchloratebeing present in an amount of about 71 weight percent of saidcomposition; said aluminum metal fuel is present in an amount of about16 weight percent of said composition; said burning rate catalyst ofn-butylferrocene is present in an amount of about 2.5 weight percent ofsaid composition; said iron linoleate is present in an amount of about0.1 weight percent of said composition; said binder is present in anamount of about 10.4 weight percent of said composition said binderbeing comprised of said polybutadiene prepolymer in an amount of about10 weight percent of said composition and crosslinked with the selectedsiloxane compound which is 1,1,3,3-tetramethyldisiloxane, and saidselected siloxane compound being present in an amount of about 0.4weight percent of said composition; and wherein said interfacial bondingagent is present in an amount of about 0.1 weight percent of the totalpropellant ingredients.
 4. The propellant composition of claim 2 andwherein said inorganic oxidizer ammonium perchlorate is a blend of 200micron particle size, 67 parts, and 20 micron particle size, 33 parts;said aluminum metal fuel is present in an amount of about 16 weightpercent of said composition; said burning rate catalyst ofn-butyl-ferrocene is present in an amount of about 2.5 weight percent ofsaid composition; said iron linoleate is present in an amount of about0.1 weight percent of said composition; said binder is present in anamount of about 10.4 weight percent of said composition, said binder iscomprised of said polybutadiene prepolymer in an amount of about 10weight percent of said composition and crosslinked with the selectedsiloxane compound which is 1,1,1,3,5,7,7,7-octamethyltetrasiloxane, saidselected siloxane compound being present in an amount of about 0.4weight percent of said composition; and wherein said interfacial bondingagent is present in an amount of about 0.1 weight percent of the totalpropellant ingredients.
 5. The composite propellant composition of claim1 wherein said composition contains an additional binder ofhydroxyl-terminated polybutadiene crosslinked with toluene diisocyanate.6. The propellant composition of claim 5 and wherein said inorganicoxidizer ammonium perchlorate is present in amounts from about 60 toabout 72 weight percent of said composition; said aluminum metal fuel ispowdered and said aluminum powder is present in amounts from about 5 toabout 20 weight percent of said composition; said burning rate catalystof n-butylferrocene is present in amounts from about 0.5 to about 3.0weight percent of said composition; said iron linoleate is present inamounts from about 0.1 to about 0.3 weight percent of said composition;said hydroxyl-terminated polybutadiene is present in amounts from about5 to about 10 percent by weight of said composition; said toluenediisocyanate is present in amounts from about 0.5 to about 2.0 percentby weight of said composition; said polybutadiene prepolymer is presentin amounts from about 5 to about 10 percent by weight of saidcomposition; said selected siloxane compound is selected from the groupconsisting of 1,1,3, 3-tetramethyldisiloxane and1,1,1,3,5,7,7,7-octamethyltetrasiloxane and is present in amounts fromabout 0.3 to about 2.0 percent by weight of said composition; and saidcomposition additionally containing an interfacial bonding agent, as anadditive, in the amount of from about 0.05 to about 1.0 percent byweight of the total propellant ingredients.
 7. The propellantcomposition of claim 6 and wherein said inorganic oxidizer ammoniumperchlorate is a blend of 200 micron size, 67 parts, and 20 micronparticle size, 33 parts, said blended ammonium perchlorate being presentin an amount of about 66 percent by weight of said composition; saidaluminum metal fuel is present in an amount of about 16 percent byweight of said composition; said burning rate catalyst ofn-butylferrocene is present in an amount of about 2.5 percent by weightof said composition; said iron linoleate is present in an amount ofabout 0.1 percent by weight of said composition; said polybutadieneprepolymer is present in an amount of about 7 percent by weight of saidcomposition; said selected siloxane compound is1,1,3,3-tetramethyldisiloxane which is present in an amount of about 0.4percent by weight of said composition; said hydroxyl-terminatedpolybutadiene is present in an amount of about 7 percent by weight ofsaid composition; said toluene diisocyanate is present in an amount ofabout 1.0 percent by weight of said composition; and said interfacialbonding agent is present in an amount of about 0.1 percent by weight ofthe total propellant ingredients.